1. Field of the Invention
The present invention relates to the field of the biological treatment of waters and relates most especially to a process for denitrifying water by means of micro-organisms employing carbon-containing substrates (denitrification by a heterotrophic route).
2. Description of the Prior Art
Biological denitrification is a well known process which begins to be applied industrially in the treatment of waters which are to be made potable.
Autotrophic denitrification is carried out in an anoxic medium through the intermediacy of bacteria such as Thiobacillus denitrificans which employ the oxygen from the NO.sub.3.sup.- ion to oxidize the sulfur.
In contrast, the reduction of nitrates by a heterotrophic route requires the presence of a carbon-containing substrate. Industrial lines make use of ethanol or acetic acid as a reducer.
Tests carried out on some soil samples have shown the ability of soil to denitrify waters in the presence of organic substances of vegetable origin, such as lucerne or straw (the use of fresh straw is preferable to that of straw which is already composted).
The composition of the gaseous mixture produced is variable, molecular nitrogen appearing to increase with the quantity of nitrates. In partly submerged soil, in the presence of straw, the disappearance of nitrates is very fast (2 days). In all these tests it appears that the reduction of nitrites is slower and the adaptation time of the micro-organisms is seen as a growth in nitrates at the beginning of the experiment.
In order to valorise the by-products of the agricultural and food industry, studies have already been made on the denitrification of waste water with harvest residues (straw or wood shavings). It has been possible, in this way, to obtain an average reduction of 40% in the NO.sub.3 content.
Nevertheless, these earlier experiments have not gone beyond the laboratory stage and, so far as the Applicant is aware, no process has hitherto been proposed which is simple and easy to use, permitting biological denitrification of waters by an in situ treatment, in order to make it possible to make waters from subterranean tables potable. The subject of the invention is such a process, in which the treatment is carried out at the catchment site of the potable water.
The patents FR-A-2,302,279 and DD-A-140,031 may be mentioned as documents illustrating the prior art in the field of the denitrification of waters.
Patent FR-A-76/05628 relates to a process for removing the nitrogen compounds present in the water, and for this purpose proposes to make use of the micro-organisms intended for the denitrification of the waters, these micro-organisms being capable of being introduced into the soil. In such a process, the denitrifying micro-organisms are introduced into the water to be treated. Similarly, the latter has added to it a liquid nutritive carbon-containing substance, for example a solution of glucose, the water to be introduced into the soil being moreover made alkaline by means of an alkali metal hydroxide, in the case where the soil has an acid reaction. It should also be noted that the denitrifying mixture is introduced directly into the aquifer. The denitrification reaction is therefore produced within the aquifer itself. Patent FR-A-76/05,628 therefore describes a denitrification process which makes use of the sub-soil as a reaction environment, but which does not make it possible to make the water potable, that is to say to obtain a water whose chemical composition conforms to the potability standards. Furthermore, the micro-organisms or bacteria which can be used for the denitrification are introduced with the water, which therefore requires an artificial enrichment with denitrifying bacteria of the water to be treated.
Patent DD-A-140,031 relates to the denitrification of stagnant waters originating, for example, from lakes and ponds. The process described in this patent consists in installing a denitrification reactor at the bottom of the water. This reactor contains a support for the bacteria, for example straw. The latter is used as a filling material and not as a carbon-containing substance which is nutritive for the micro-organisms, this substance being a liquid substance introduced with the water. Moreover, patent DD-A-140,031 suggests the replacement of the body filling the reactor by a biologically inert material. There is no teaching in this prior document of a process which makes it possible to treat subterranean waters directly to obtain a potable water by making direct use of the micro-organisms or bacteria present in the sub-soil, and without it being necessary to employ a reactor situated in the sub-soil.